1. Field of the Invention
The present invention relates to a developing apparatus, to a developing apparatus in which the developing roller is used, and to a process cartridge and an image formation apparatus equipped with the developing apparatus.
2. Description of the Related Art
With an electrophotographic image formation apparatus such as a copier, laser printer, or fax machine, or a multi-purpose machine combining two or more of these functions, an electrostatic latent image formed on a latent image support such as a photosensitive drum or photosensitive belt is developed by a developing apparatus to produce a visible image. A so-called two-component developing system, featuring a developer obtained by mixing a nonmagnetic toner with a magnetic carrier, is well known and has been widely used in such developing apparatus in recent years.
With this two-component developing system, the developer is magnetically held to the outer peripheral surface of a developing roller to form a magnetic brush, and an electrostatic latent image is developed in a developing region where there is an electrical field sufficient for developing between the developing roller and the latent image support, by selectively supplying toner and causing it to adhere to the latent image on the latent image support across from the magnetic brush by means of the electrical field formed between the latent image support on which the electrostatic latent image has been formed and a sleeve to which an electrical bias has been applied.
A developing roller is generally equipped with a cylindrical developing sleeve composed of a nonmagnetic material, and a magnet roll is provided inside this sleeve so as to form a magnetic field that will cause the developer to rise in the form of a magnetic brush on the rear surface of the sleeve. With a developing roller such as this, the carrier rises on the sleeve along the magnetic lines of force issuing from the magnet roll, and charged toner is deposited on the resulting carrier. The magnet roll has a plurality of magnetic poles formed from magnets or the like, and is equipped with a developing pole for raising the developer, particularly in the developing region portion of the sleeve surface. When the developing sleeve and/or the magnet roll moves, the developer that has risen in the form of a magnetic brush on the sleeve surface also moves, the developer conveyed to the developing region is raised up along the magnetic lines of force issuing from the developing main pole, forming brush chains, these developer chains bend while coming into contact with the latent image support surface, and toner is supplied while the brush chains rub against the electrostatic latent image on the basis of a difference in relative linear velocity versus the latent image support.
With a conventional two-component developing type of developing apparatus, the developing conditions for raising image density are incompatible with the developing conditions for obtaining an image with good contrast, making it difficult to improve both a high density portion and a low density portion at the same time. Examples of developing conditions for raising image density include narrowing the developing gap (the gap between the latent image support and the developing sleeve), and broadening the developing region in width. Meanwhile, examples of developing conditions for obtaining an image with good contrast include widening the developing gap, and narrowing the developing region width. In other words, these two developing conditions are contradictory, and it is generally difficult to obtain a good-quality image by satisfying both conditions over the entire image density range.
For instance, when the emphasis is on obtaining a low-contrast image, the trailing edge of a black solid image or a halftone solid image tends to be lost, which also occurs with the crossing portions of solid lines.
Raising the magnetic flux density of the developing pole and narrowing the half value width is an effective way to reduce this trailing edge loss. Various constitutions in which a molded magnet with high magnetic characteristics is disposed at a location corresponding to the developing pole of a magnet roll have been proposed in the past in an effort to achieve high magnetic flux density and narrow half value width, one of which is disclosed in Japanese Laid-Open Patent Application 2001-296743, for example.
With the image formation apparatus of recent years, however, there has been a trend toward reducing the particle size of the developer carrier because of the need for higher image quality. Nevertheless, when the particle size of the carrier is reduced, there is less margin for carrier deposition on the latent image support with the developing roller discussed in the above-mentioned publication, and the carrier tends to be deposited along with the toner on the latent image support. “Carrier deposition” refers to a phenomenon whereby the carrier which is supposed to accumulate on the developing roller is deposited on the latent image support along with the toner in the course of the developer being conveyed for developing to the latent image support by the magnetic force of the developing roller. This is a product of the balance between the electrical force from the latent image support and the magnetic force from the developing roller acting on the carrier. If the electrical force is strong, the carrier will be deposited on the latent image support. The deposited carrier is transferred and fixed along with the toner on the paper, which has an adverse effect on the transfer apparatus and fixing apparatus, and is a cause of lower reliability of an image formation apparatus. In order to prevent carrier deposition, the charge potential of the latent image support or the potential of the developing roller is sometimes adjusted so as to reduce the electrical force to which the carrier is subjected, but this tends to result in image problems such as greasing.